Broadband absolute radar signature measurements as well as image diagnostic analyses are necessary for developing and proving technology that minimizes the return of radar energy from military vehicles such as aircraft, missiles, ground vehicles and ships. Verification of the low observable performance of such vehicles is important, not only at production acceptance but also after depot level repair, flight line maintenance and battle damage repair.
Heretofore, verification of low observable performance after flight line maintenance and after battle damage repair has been performed by small, surface sensors, that only give an indication of the relative quality of the repair and do not provide verification of the overall low observable performance of the vehicle. The standard facilities for low observables research and development testing are either large outdoor test facilities or specially constructed indoor compact ranges. An outdoor range requires a large amount of controllable flat real estate, which must be remote from centers of population for security reasons and to avoid electromagnetic interference. The large size of an outdoor range is needed for the production of uncluttered far field conditions. That is, the target whose radar signature is to be determined, and the radar must be separated by several thousand feet so that the target illumination can be approximated by an uniform planar wave. Outdoor measurement facilities cost on the order of $80 million to construct. They also have the disadvantage that in many instances, the target under test must be mounted on a pylon in the open, which means that over flying satellites can acquire optical views of the target and can determine the radar frequencies at which the target is being tested. The only effective counter measure for such fly over satellites is to operate such ranges only when no foreign satellite is in view, which means a great deal of the available range time is wasted shielding the target from view.
Indoor compact ranges do not require the vast amount of real estate that outdoor ranges require. However, they require the fabrication and deployment of large parabolic reflectors to simulate the far field plane wave test conditions. The reflector and facility construction costs unfortunately expand exponentially with target size. Hence, indoor compact ranges are economic for testing small targets, but ranges that can test large targets up to about 40 feet, typically cost about $30 million or more. Indoor ranges have the advantage over outdoor ranges, however, that security is more easily maintained and they can be operated continuously 24 hours a day because their operations are shielded from spy satellites, and protected from weather.
However, there has been a need to provide an economic test facility capable of measuring the radar signature of large low observable targets as accurately as existing outdoor range and indoor compact range facilities, but with a total cost to implement in existing buildings of about a tenth of the cost for an outdoor range, or a comparable indoor compact range.